The M. Shah, U.S. Pat. No. 5,463,493 (hereinafter referred to as the '493 patent), entitled: "Acousto-Optic Polychromatic Light Modulator," describes a polychromatic acousto-optic modulator (PCAOM) that is configured to overcome a number of hardware complexity and operational deficiencies of color laser projection systems employing multiple `single-color` modulators. For this purpose, the polychromatic acousto-optic modulation scheme employed in the '493 patent uses an acousto-optic medium and a single piezo-electric transducer attached to the acousto-optic medium to convert a multi-RF frequency input signal into ultrasonic waves for modulating a multi-wavelength input light beam. An electronic driver supplies electrical signals containing a plurality of different RF frequency components, the intensities of which are controlled in response to input electronic data, so as to produce a desired multi optical frequency (color) beam output from the diffracted polychromatic beam.
Where it is desirable that the polychromatic modulator produce a single highly convergent polychromatic output beam from a randomly polarized input beam, the '493 patent describes a modulator configuration diagrammatically illustrated in FIG. 1, which effectively corresponds to FIG. 11 of the '493 patent. As generally described in column 8, lines 43-67, and column 9, lines 1-22, and more particularly, in lines 23-47 of the '493 patent, for such an embodiment, a randomly polarized polychromatic input light beam 110 traveling in a direction having an angle Ci relative the transducer bonding face 41 is first spatially separated into two orthogonally polarized beams 111 and 112, using a birefringent plate as an input beam polarization separation interface.
The entrance face of the modulator body (the acousto-optic medium, such as a TeO.sub.2 crystal) 40 is cut with two facets 115 and 116 with respective angles .alpha..sub.i.sup.1 and .alpha..sub.i.sup.2 from the bonding face 41 of a transducer 43 to provide independent angles for the two polarization states produced by the birefringent plate 114. The RF frequencies and drive levels applied over input signal lines 47 to an RF driver 45 are selected to cover all desired wavelengths and its polarizations. The exit face of the modulator body 40 is also cut with two facets 117 and 118 at respective angles at .alpha..sub.i.sup.1 and .alpha..sub.i.sup.2 from the bonding face 41 to provide independent exit angles for each polarization. An output beam polarization combining interface in the form of a birefringent plate 119 is positioned at the output side of the modulator body 40, in order to recombine the two orthogonally polarized polychromatic output beams into one convergent beam 120. If the desired output consists of two orthogonally polarized individually color convergent separated beams, then the birefringent plate 119 is eliminated.
The '493 patent further states that calculations show that at Ci=55.degree., .alpha..sub.i.sup.1 =0.degree., .alpha..sub.o.sup.1 =87.93.degree., .alpha..sub.i.sup.2 =92.07.degree., and .alpha..sub.o.sup.2 =0.degree. in far-off-axis mode TeO.sub.2, the first order diffracted beams for each polarization emerge the modulator body in precisely the same direction as the incident optical beam, and the RF drive frequency for any selected wavelength within the visible range is independent of the polarization state.
A shortcoming of this configuration is the fact that it is, in reality, a polarization sensitive device, so that it requires a hardware intensive architecture. Namely, the prior art device of FIG. 1 installs a polarization separation interface (birefringent plate 114) in the path of the optical input beam 110 upstream of the modulator body 40, in order to provide two orthogonally polarized input beams upon which the modulator body may operate. In order to interact with these spatially separated and orthogonally polarized beams, the modulator body itself is fashioned as a specially cut crystal body with differentially cut entrance and exit facets.